VPN Tunnelling and what Microsoft uses (PPTP)

Virtual Private Networks (VPNs) offer a cost effective alternative to other mechanisms to connect geographically distance offices such as leased lines (sometimes called "private circuit"), which involves paying a monthly lease to a provider in exchange for a permanent connection between two locations. A problem can be lack of guaranteed bandwidth which you get with a leased line.

The first form of VPNs were software defined networks (SDNs) developed by AT&T in the 1980s.

Today's technology relies on tunnelling protocols, the main ones being:

• IP Security (IPSec) which is an open standard that deals with authentication and security of transmission over the public network.
• PPTP or Point to Point Tunnelling Protocol, developed by an industry consortium including Microsoft and 3Com is used by Windows.
• Layer 2 Tunnel Protocol (L2TP) was developed by Cisco to replace IPSec

Tunnelling offers the safety and security required to make VPNs a viable proposition. It behaves like an envelope for letters - the data packet in the payload of another packet which follows another protocol. This technique also allows routing of non IP packets.

The tunneled packet header contacts a bunch of information relating to the protocol of the enclosed packet, the initiator and potentially a checksum to ensure data was not corrupted during transmission.